OLED luminance degradation compensation

• US 8,026,876 B2
• Filed: 08/15/2007
• Issued: 09/27/2011
• Est. Priority Date: 08/15/2006
• Status: Active Grant

• Current Assignee: IGNIS Innovation Incorporated
• Original Assignee: IGNIS Innovation Incorporated
• Inventors: Chaji, G. Reza, Nathan, Arokia
• Primary Examiner(s): Eisen; Alexander
• Assistant Examiner(s): Marinelli; Patrick
• Application Number:
  US11/839,145
• Publication Number:
  US 20080088648A1
• Time in Patent Office: 1,504 Days
• Field of Search: 345 76- 83, 345204-215, 3151691-1694
• US Class Current: 345/78
• CPC Class Codes: G09G 2300/0465 Improved aperture ratio, e.g. by size reduction of the pixel circuit, e.g. for improving the pixel density or the maximum displayable luminance or brightness
• CPC Class Codes: G09G 2300/0819 used for counteracting undesired variations, e.g. feedback or autozeroing
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2320/0233 Improving the luminance or brightness uniformity across the screen
• CPC Class Codes: G09G 2320/029 by monitoring one or more pixels in the display panel, e.g. by monitoring a fixed reference pixel
• CPC Class Codes: G09G 2320/0295 by monitoring each display pixel
• CPC Class Codes: G09G 2320/041 Temperature compensation
• CPC Class Codes: G09G 2320/043 Preventing or counteracting the effects of ageing
• CPC Class Codes: G09G 2320/045 Compensation of drifts in the characteristics of light emitting or modulating elements
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element
• CPC Class Codes: G09G 3/3241 the current through the light-emitting element being set using a data current provided by the data driver, e.g. by using a two-transistor current mirror
• CPC Class Codes: G09G 3/3283 in which the data driver supplies a variable data current for setting the current through, or the voltage across, the light-emitting elements
• CPC Class Codes: H05B 45/60 Circuit arrangements for operating LEDs comprising organic material, e.g. for operating organic light-emitting diodes [OLED] or polymer light-emitting diodes [PLED]

Abstract

A system and method are disclosed for determining a pixel capacitance. The pixel capacitance is correlated to a pixel age to determine a current correction factor used for compensating the pixel drive current to account for luminance degradation of the pixel that results from the pixel aging.

Claims

1. A method of compensating for luminance degradation of a pixel having an electroluminescent device, the method comprising:

determining the capacitance of the electroluminescent device;

correlating the determined capacitance of the electroluminescent device to a current correction factor for the electroluminescent device;

compensating a drive current for the electroluminescent device according to the correlated current correction factor; and

driving the electroluminescent device with the compensated drive current;

wherein the step of determining the capacitance of the electroluminescent device comprises;

charging the capacitance of the electroluminescent device to a first voltage V1;

charging a parasitic capacitance to a second voltage V2;

electrically connecting the parasitic capacitance and the capacitance of the electroluminescent device in parallel;

and measuring a voltage change, Δ

V, across a read capacitor of capacitance C_read;

wherein the capacitance of the electroluminescent device is equal to;

2. The method as claimed in claim 1, wherein the capacitance of the electroluminescent device and the parasitic capacitance are electrically connected in parallel during the charging of the capacitance of the electroluminescent device to V1, and the capacitance of the electroluminescent device and the parasitic capacitance are electrically disconnected during the charging of the parasitic capacitance to V2.

3. The method as claimed in claim 2, further comprising:

determining a leakage current of the read capacitor prior to measuring Δ

V;

determining a resultant voltage based on the leakage current; and

deducting the resultant voltage from Δ

V.

4. The method as claimed in claim 1, wherein the electroluminescent device is one of a plurality of electroluminescent devices arranged in an array to form a display.

5. A method of driving a pixel with a current compensated for luminance degradation of the pixel, the method comprising:

determining the capacitance of the pixel;

correlating the determined capacitance of the pixel to a current correction factor for the pixel;

compensating a pixel drive current according to the current correction factor;

and driving the pixel with the compensated pixel drive current;

wherein the step of determining the capacitance of the pixel comprises;

charging the capacitance of the pixel to a first voltage V1;

charging a parasitic capacitance to a second voltage V2;

electrically connecting the parasitic capacitance and the capacitance of the pixel in parallel;

and measuring a voltage change, Δ

V, across a read capacitor of capacitance C_read;

wherein the capacitance of the pixel is equal to;

6. A display for driving an array of a plurality of pixel circuits with a current compensated for luminance degradation, each of said pixel circuits having an electroluminescent device, the display comprising:

a display panel comprising the array of pixel circuits, the pixel circuits arranged in at least one row and a plurality of columns;

a column driver for driving the electroluminescent device in each pixel circuit of the plurality of pixel circuits with a driving current;

a read block for determining the capacitance of an electroluminescent device and correlating the determined capacitance of the electroluminescent device to a current correction factor for the electroluminescent device; and

a control block for controlling the operation of the column driver and the read block, the control block being operable to compensate the driving current based on the correlated current correction factor, and to drive the electroluminescent device with the compensated driving current;

wherein the step of determining the capacitance of the electroluminescent device comprises;

charging the capacitance of the electroluminescent device to a first voltage V1;

charging a parasitic capacitance to a second voltage V2;

electrically connecting the parasitic capacitance and the capacitance of the electroluminescent device in parallel;

and measuring a voltage change, Δ

V, across a read capacitor of capacitance C_read;

wherein the capacitance of the electroluminescent device is equal to;

7. The display as claimed in claim 6, further comprising:

at least two rows of pixel circuits; and

a row driver for selecting the row of pixel circuits to be driven by the column driver.

8. The display as claimed in claim 7, wherein each pixel circuit comprises:

an electroluminescent device for emitting light based on the driving current; and

a switching transistor, controlled by the row driver for controlling a driving transistor, the driving transistor for driving the electroluminescent device based on the driving current.

9. The display as claimed in claim 8, wherein the electroluminescent device is an organic light emitting diode.

10. The display as claimed in claim 8, wherein the read block comprises:

a plurality of read block elements, each read block element comprising;

a switch for electrically connecting and disconnecting the read block element to a pixel circuit of the plurality of pixel circuits;

an operational amplifier electrically connected to the switch; and

the read capacitor connected in parallel with the operational amplifier.

11. The display as claimed in claim 6, wherein each pixel circuit comprises:

a transistor for controlling the driving current from the column driver; and

an electroluminescent device for emitting light based on the driving current.

12. The display as claimed in claim 11, wherein the electroluminescent device is an organic light emitting diode.

13. The display as claimed in claim 11, wherein the read block comprises:

a plurality of read block elements, each read block element comprising;

a switch for electrically connecting and disconnecting the read block element to a pixel circuit of the plurality of pixel circuits;

an operational amplifier electrically connected to the switch; and

the read capacitor connected in parallel with the operational amplifier.

14. The display as claimed in claim 6, wherein the control block operates the display in one of at least two modes:

a display mode wherein the control block controls the current driver for driving the plurality of pixel circuits with a driving current based on a display signal and the current correction factor, to emit light;

and a read mode wherein the control block controls the read block to determine the capacitance of the electroluminescent device of a pixel circuit of the plurality of pixel circuits, the control block determining the current correction factor based on the capacitance of the electroluminescent device of the pixel circuit.